This invention relates to front fork, telescoping type suspension systems for bicycles. The invention is comprised of an improved fork crown, a brake arch structure and attaching means to other fork components. The crown structure is that part of the suspension that connects the stanchion tubes (the upper part of the telescoping assembly) to the steerer tube. The brake arch attaches the upper portion of the right sliding fork leg or strut to the upper portion of the left sliding fork leg or strut, as well as supporting the brake cable stop and brake caliper assembly (in the case of cantilever brakes). The invention provides a simple and inexpensive means for reducing the overall fork weight and at the same time improving the bending and torsional stiffness and strength of the overall fork, and specifically the fork crown and brake arch components.
The invention provides an improved method for mounting the stanchion tubes to the fork crown and lower fork tubes to the brake arch, using a collet, wedge, pinch bolt or bonded assembly. Except in the case of the bonded assembly, these methods allow quick assembly and disassembly of the suspension system, for repairs and parts replacements.
In the design of competition bicycles and bicycle parts, weight and stiffness are critical issues. Extremely lightweight structures and structural components are used in the most serious competition bicycles. These lightweight components must be designed for a variety of severe riding environments. This results in a design that must operate at relatively high stresses, close to the strength limits of the materials being used. The demand for a minimum weight bicycle has led the industry into the use of modern, high performance structural materials, such as high strength aluminum, carbon fiber composite and titanium alloys. These high strength materials require more care in the design of fittings and joints because of, a) their susceptibility to fatigue cracking and b) the relatively high load levels at which the fittings and joints are required to operate.
A goal for a bicycle part manufacturer is to eliminate all unnecessary weight from a given part, without compromising its structural integrity and stiffness. There are numerous bicycle suspension forks currently on the market that are not very weight efficient. They have been designed for basic suspension function, without adequate consideration for weight optimization or steering and braking control. Most of the prior art telescoping front fork suspensions fall into this category. These designs tend to be relatively heavy and their stiffness to weight and strength to weight ratios are not very high. They are also relatively flexible laterally and in torsion and cannot provide the stability and accurate steering and braking control for the front wheel assembly that is desired for serious competition cycling. Laboratory tests show that some of the prior art fork designs have torsional spring rates as low as 84 in-lbf/deg and lateral spring rates as low as 140 lbf/in. Some of the heavier steel forks have torsional spring rates in the neighborhood of 230 in-lbf/deg and lateral spring rates of nearly 170 lbf/in, however, their weight exceeds 1500 grams. Based on studies, it has been found that a torsional spring rate in excess of 230 in-lbf/degree and a lateral spring rate in excess of 170 lbf/in is desirable for maximum steering control in competition cycling. The weight of the suspension should be less than 1000 grams.
Most of the prior art fork suspensions use brake arch designs that are inherently too flexible to control wheel wobble and braking action. The name xe2x80x9cbrake bridgexe2x80x9d or xe2x80x9cbrake archxe2x80x9d says it all. The part was designed and located simply as a support for the brake cable hanger and possibly the brake mounts, similar to the part of the same name used on the rear seat stays of the bicycle. The prior art designs did not realize that the lower sliding tubes need to be rigidly linked to each other in torsion and bending in order to provide top performance of the cantilever brakes and the overall suspension fork assembly.
The present invention uses a unique design for the separate crown structure and brake arch assembly to dramatically increase the strength and stiffness of the fork while reducing weight. The crown structure and the brake arch play key parts in the overall stiffness of the front fork assembly. The invention also provides an improved method of assembly of the various key parts of the suspension fork to reduce manufacturing costs as well as make the system easier to assemble and disassemble for parts and repairs.
Prior art front fork suspensions come in a variety of sizes and shapes as shown in FIGS. 1-5 (common components are identified by a numeral preceded by the figure number). Most of the more popular designs use telescoping struts, operating pneumatically, hydraulically, elastomerically or with metallic springs to achieve the suspension action. FIG. 1 illustrates the structural arrangement typical of these designs. For example, some forks utilize a unicrown 3 type of construction (used on many non-suspended forks), consisting of a tight bend created in the top of the fork blade, where the fork blade is directly attached, through brazing, welding or other means to the steerer tube 2. FIG. 2 illustrates the unicrown type fork design. The integral blade and crown form the upper tube of the suspension. This type of construction has the advantage of not having a separate fork crown, but it also has several disadvantages. The curved tube upper structure coincides with the most highly stressed region of the fork. Under stress, the essentially round or elliptical sections deform significantly out of round, creating excessive movement and stress concentrations. The process of bending the curve into the tube stretches and thins the outer wall of the tube, weakening it. The welded or brazed joint to the steerer is a weakened area as a result of the thermal effects, residual stress and stress risers due to the joint configuration. The bent and welded type of construction does not lend itself to a highly accurate alignment of the two upper fork blades, or stanchion tubes, which make it difficult to make a high precision sliding structure.
For the separate fork crown member type of design (FIG. 1), the stanchion tubes (the stationary part of the telescoping assemblyxe2x80x94Items 1L and 1R) are connected to the steerer tube (2), by a common crown part (3). The crown is typically made of aluminum alloy, either machined out of solid or forged, with subsequent machining of the steerer and stanchion tubes fitting surfaces. In prior art, the stanchion tubes are retained by adhesive, interference fit or pinch bolts, or a combination of the above. The structural support between the steerer tube and the stanchion tube is typically either a solid rectangle or inverted channel shape.
Generally, one of the most critical and highly loaded parts of the suspension fork design is the crown structure (3). This part must be designed to handle both bending and torsional loads resulting from frontal and side impacts to the wheel. The crown acts as a structural transfer member to transmit the impact loads to the steerer where these loads are distributed to the head-set bearings and eventually to the bicycle frame.
Also, very important to the stability and performance of a telescoping type suspension fork is the brake arch or brake bridge, as it is sometimes called. The brake bridge connects the two struts and causes them to telescope together during wheel impact, thereby minimizing wheel xe2x80x9cwobblexe2x80x9d. If the two telescoping tubes are allowed to move independently, the wheel will wobble and create high stresses at dropout/axle connection. Neither condition is desirable. The brake bridge provides resistance against the up-down, for-aft and rotational (torsional) movements of the struts, forcing the wheel to run true during full suspension travel. The brake bridge (4) also serves as a structural support for the brake cable stop (6).
There are several configurations that are currently used for the brake bridge. FIG. 1 shows the configuration for one of the leading fork designs. This design uses an arched beam having a variable rectangular section. At the end connections the section is basically a hollow rectangular tube. Near the top of the arch the section becomes solid. In between the height of the section is constant but the width is variable. The ends of the beam are connected by fasteners.
FIG. 3 illustrates a plate type arch which is sometimes used in prior art designs. This concept is inherently weak in out-of-plane bending and torsion and tends to be heavy.
FIG. 4 illustrates a small diameter tube type arch which have sections that are too small to provide the rigidity needed. Usually these designs have tube diameters less than half the diameter of the stanchion tubes. These designs are relatively flexible in torsion.
FIG. 5 illustrate an I-beam arch that is currently used. I-beams are inherently weak and flexible in torsion and cannot really function properly for this application. The invention provides for substantially increased stiffness and lower stresses in the brake arch, which translates into improved directional stability for the front wheel, less displacement for the brake cable stop (when the brake loads are applied) and improved fatigue life for the assembly. The tube diameter for the arch is roughly the same as the stanchion tubes, giving it significantly more torsional and bending stiffness than prior art designs. The invention also provides for a simpler and less expensive means for mounting the brake bridge to the fork structure.
The performance of a telescoping type of suspension front fork is similar to a chain, that is, it is only as strong as the weakest link. The stiffness of a structure is not additive, the deflections of a structure under load are. If three load bearing parts of a fork are quite stiff, but two load bearing parts deflect greatly, the overall deflection will be large because of the flexible parts.
This is the case with prior art forks. Because of strength, weight and economic considerations, the steerer tubes, stanchion tubes and lower sliding tubes are typically of adequate strength and relatively light weight. The steerer tube is highly loaded in bending, and is typically adequate in torsion and bending stiffness. It only sees the torsion loads involved with steering. The stanchion tubes see predominantly large bending stress, and are typically of adequate stiffness in this mode as well. The lower sliding tubes are typically larger in diameter than the stanchion tubes as they house the sliding bearings and fit over the typical stanchion tube, and as such typically have considerable inherent stiffness.
It is easy to design and manufacture a straight wall or butted tube with good properties. The brake arch (and to a lesser degree the fork crown), on the other hand, need much more stiffness than their strength requirements dictate. The only thing tying the lower fork legs together besides the brake arch in most telescoping type forks in the front wheel axle. As a lateral force is applied to the wheel in contact with the ground, such as in cornering, the lower fork blade on one side is compressed and the other one is extended. Only the wheel axle and the brake arch resist this shearing action.
When the cantilever brakes are applied in stopping, they push outward and also put a large amount of torsion on the two lower fork tubes. Since the lower fork blades are free to twist on the stanchion tubes, the resistance to the torsion is provided by the wheel axle and the brake arch. The outward pushing force will create additional stress on the sliding bearing assembly, substantially increasing the sliding friction. This has the result that the fork suspension will not work freely while braking. It tends to xe2x80x9clock upxe2x80x9d.
When steering forces are applied to the front wheel, as when trying to steer the front wheel out of a rut, the lower sliding assembly of the fork will twist, as will the fork crown, reducing the riders control. When the wheel does not follow in the direction the rider has steered the handlebars, a crash is often the result.
Some companies have tried to increase the stiffness of the overall assembly by increasing the axle diameter on special xe2x80x9csuspensionxe2x80x9d hubs from 9 mm to 10, 11 or even 12 mm in diameter. This is still relatively small, and coupled with the not totally rigid quick release wheel/dropout joint, does not provide the additional rigidity that the lower sliding portion of the suspension forks need.
It appears that some products have realized the nature of the flex problem, as there are aftermarket reinforced brake arches to improve the stock suspension forks. These reinforced arches have a typical flat plate or light weight I beam construction, often with lightening holes drilled through it. The companies have picked up on the need for shear resistance, but have not addressed the torsion rigidity needs. The modified arches still bolt onto the lower fork blades in the original non rigid manner.
The present invention uses a unique design for the brake arch to increase its strength and stiffness an order of magnitude with little or no increase in weight. The invention also provides an improved method for mounting the brake bridge to the lower fork structure. This helps to reduce manufacturing costs as well as make the system easier to assemble and disassemble for parts and repairs. The improved method also results in much higher rigidity through the joint.
Prior art designs for the brake arch tend to be relatively flexible, due to their small section geometry and poor end connections. This allows the wheel assembly to move from side to side during severe side and vertical bump loadings. Also, these designs, because of the bridge layout and section geometry, requires that the brake cable stop (6) be cantilevered quite a distance from the axis of the bridge. This introduces higher torsional and bending stresses in the bridge and greater displacements at the brake cable stop (6), when the brake loads are applied. This invention overcomes these difficulties by introducing a superior section geometry and more substantial end connections for the bridge. This adds significantly more flexural and torsional rigidity to the bridge structure. Also, because of the larger section geometry of the brake bridge, the brake cable stop (6) is more in line with the bridge axis (less cantilever action) thereby reducing the local bending and torsional stresses from the brake cable stop loads. The end connections of the bridge are also designed for ease of assembly and disassembly, in the case of the bolted, wedge or collet versions.
The objectives of this invention are: a) minimize the weight of the entire suspension fork assembly, b) create a xe2x80x9cstiffxe2x80x9d suspension structure where the wheel motion is restricted to the desired vertical travel only, and cannot move laterally or torsionally in the fork. c) Maximize the lateral stiffness of the entire fork, d) increase the yield and fatigue strengths of the fork and its attachments, e) reduce the bending and torsional deflection of the brake arch structure and brake attachments from braking and f) improve the method of assembling the bridge structure and the fork crown to the fork.